This invention relates to impact resistant compositions and a process therefor. More particularly, the invention relates to impact resistant styrene polymer blends and the production thereof at high rates.
It is known that the impact strength and other physical properties of styrene polymers can be improved by combining such polymers with compositions containing a rubber. Although maximum increases in impact strength are generally exhibited by rubber-modified styrene polymers such as are prepared by interpolymerization of a rubber and styrene monomer, impact resistant compositions also may be prepared by blending certain compositions containing a rubber with styrene polymers. Exemplary of prior art efforts to prepare such impact resistant blends is U.S. Pat. No. 3,637,554 to Childers wherein improvements in impact strength and other physical properties are achieved by blending with a commercial polystyrene resin, a branched block copolymer having a polyfunctional nucleus to which are bonded from three to seven chains of a block copolymer prepared from a conjugated diene and a vinyl substituted aromatic compound.
U.S. Pat. No. 3,781,383 to Finestone et al. discloses impact resistant rubber-modified styrene polymers and the blending of such polymers with styrene homopolymers to obtain impact resistant blends. Although it is shown that the impact strengths of the disclosed rubber-modified styrene polymers exceed the impact strengths of blends containing such a rubber-modified polymer and a styrene homopolymer, blending can be carried out without losing the effectiveness of the rubber-modified material as a toughening agent and it is possible to prepare blends of predetermined impact strength. The disclosed rubber-modified styrene polymers are prepared by mass polymerizing a polymerization feedstock comprising a solution of rubber in styrene monomer with the addition of at least 0.005 wt. % of a mercaptan chain transfer agent to the feedstock when about 2 to 15 wt. % of the monomer has been converted to polymer.
U.S. Pat. No. 3,906,058 to Durst also relates to impact resistant compositions and the preparation thereof. It is taught that such compositions may be prepared by blending a first component consisting essentially of homopolymerized styrene having a number average molecular weight ranging from about 50,000 to 500,000, and a graded block copolymer of styrene and butadiene.
Prior art efforts, such as the above-described references, have resulted in impact resistant blends which are suitable for use in molding and other applications. However, difficulties are encountered in the preparation of such blends in that suitable impact strengths are often achieved only through the use of specially prepared components which add cost and time delays to the preparation of the blends. For example, certain branched block copolymers disclosed by Childers require up to 22 hours for preparation and certain rubber-modified styrene polymers employed in the blends of Finestone et al. require up to 14 hours for preparation. Moreover, the impact strengths of blended compositions are generally achieved at the expense of the impact strength of at least one of the components of the blend. Thus, for example, it is shown in Finestone et al. that blending of a rubber-modified styrene polymer having an Izod value of 2.83 ft.-lbs./inch with crystalline polystyrene results in a blended composition having an Izod value of 1.66 ft.-lbs./inch.
Accordingly, it is an object of this invention to provide impact resistant blended compositions which overcome the objections to prior art blends. It is a further object of the invention to provide a method for the preparation of such impact resistant compositions. A further object of this invention is to provide compositions having impact strengths comparable or superior to those of conventional impact resistant rubber-modified styrene polymers containing comparable amounts of rubber at production rates substantially higher than those of such conventional polymers. A still further object of the invention is to provide a method whereby a rubber-modified styrene polymer of inferior or marginal impact strength is upgraded by blending such polymer with a second styrene polymer which is also inferior or marginal in terms of impact strength. Other objects of the invention will be apparent to persons of skill in the art from the following description and claims.
It has now been found that impact resistant compositions can be prepared by blending a high molecular weight styrene polymer with a rubber-modified styrene polymer prepared by mass polymerization of a solution of a rubber in styrene monomer. The impact resistance of such blended compositions is believed to be attributable to the combination of the rubber particles of the latter component and the high molecular weight of the former component. Advantageously, it has been found that rubber-modified styrene polymers prepared at high polymerization rates, while exhibiting Izod impact strengths lower than those which would be expected of rubber-modified styrene polymers prepared at lower conventional polymerization rates using equal amounts of rubber, can be blended with high molecular weight styrene polymers to obtain, at extremely high production rates, blends having Izod impact strengths exceeding not only the respective Izod values of the components thereof, but also, the sum of such Izod values. Moreover, such blends exhibit Izod impact strengths equal or superior to those which would be expected of rubber-modified styrene polymers prepared by conventional mass polymerization techniques using rubber levels comparable to those of the blends, and the time required for preparation of such blends is substantially shorter than the time required for preparation of rubber-modified styrene polymers by conventional mass polymerization techniques.